Electrical apparatus including an improved high voltage current limiting protective device

ABSTRACT

An electrical apparatus including a protective device with an electrically insulating bushing having an axially extending inner bore and terminal members disposed adjacent to the opposite ends of the bushing. A fusible element or fuse link is disposed in and spaced from the inner bore of the bushing and is electrically connected between the terminal members. A pulverulent or granular arc quenching material substantially fills the space between the fusible element and the inner bore of the bushing which includes at its outer periphery axially intermediate the ends of the bushing a portion which is adapted to receive a supporting means for the bushing on the wall or cover of the enclosure of the associated electrical apparatus.

United States, Patent McKeithan et al.

[54] ELECTRICAL APPARATUS INCLUDING AN IMPROVED HIGH VOLTAGE CURRENT LIMITING PROTECTIVE DEVICE Inventors: Wesley L. McKeithan, Pittsburgh; John J.

Astleiord, Sharon, both of Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Dec. 10, 1969 Appl. No.:' 883,911

[73] Assignee:

US. Cl ..337/202, 337/166, 337/246 Int. Cl ..H0lh 35/14, HOlh 85/20 Field oiSearch ..337/161,166,168, 186,187, 337/201, 202, 204, 208, 209, 222, 224, 227, 228, 231, 245, 246, 159,293,295

[56] References Cited UNITED STATES PATENTS 3,287,525 11/1966 Mikulecky ..337/293X [451 Apr. 25, 1972 3,345,483 10/1967 Leonard et a1 ..337/202 X 3,309,477 3/1967 Bronikowski ..337/202 X 2,523,139 9/1950 Owens ..337/231 X 2,108,993 2/1938 Ramsey... ..337/202 1,730,716 10/1929 Austin ..337/224 UX Primary Examiner-Bernard A. Gilheany Assistant Examiner-Dewitt M. Morgan Attorney -A. T. Stratton and C. L. McHale [57] ABSTRACT An electrical apparatus including a protective device with an electrically insulating bushing having an axially extending inner bore and terminal members disposed adjacent to the opposite ends of the bushing. A fusible element or fuse link is disposed in and spaced from the inner bore of the bushing and is electrically connected between the terminal members. A pulverulent or granular arc quenching material substantially fills the space between the fusible element and the inner bore of the bushing which includes at its outer periphery axially intermediate the ends of the bushing a portion which is adapted to receive a supporting means for the bushing on the wall or cover of the enclosure of the associated electrical apparatus.

14 Claims, 2 Drawing Figures PATENTEUAPR 2 5 I972 INV RS WESLEY L. MCKEI HA 81 JOHN J. ASTLEFORD WITNES/SS F :2;

ELECTRICAL APPARATUS INCLUDING AN IMPROVED HIGH VOLTAGE CURRENT LIMITING PROTECTIVE DEVICE CROSS REFERENCES TO RELATED APPLICATIONS Certain subject matter disclosed and claimed in the present application is related to subject matter which is disclosed and claimed in copending application Ser. No. 883,951 filed concurrently by Wesley L. McKeithan and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION In the construction of certain types of electrical apparatus, such as transformers, which include a housing or enclosure in which certain portions of the apparatus are disposed, it is necessary to provide some electrically insulating means, such as an electrical bushing, having a conductor stud passing through it for carrying electrical current through the wall or cover of the enclosure to an electrical winding or other portions of the apparatus within the enclosure. In addition, it is often necessary or desirable to provide a protective device, such as a fuse, to protect such apparatus from certain abnormal operating conditions. In the past, different constructions have been proposed in which a fuse for an electrical apparatus which includes its own insulating housing or tube is assembled inside a hollow electrical bushing which is mounted on an electrical apparatus such as disclosed in U.S. Pat. No. 2,337,353 and in U.S. Pat. Nos. 3,299,235 to M. G. Leonard and 3,421,130 to M. G. Leonard et al. which are both assigned to the same assignee as the present application.

Such proposed constructions have several disadvantages with respect to the relative size of such structures and with respect to the possibility of corona in such structures particularly when applied at higher voltages, such as 5 KV and above. It is therefore desirable to provide an improved means for carrying electrical current through the enclosure orcasing of an electrical apparatus and for protecting such apparatus from certain abnormal operating conditions.

SUMMARY OF THE INVENTION In accordance with the invention, an electrical apparatus includes a protective device which also serves to carry electrical current through the wall or cover of the enclosure of the apparatus. The protective device includes a hollow, generally cylindrical electrical insulating member or bushing having an inner bore extending axially therethrough. Adjacent to the opposite ends of the bushing, terminal members or means are disposed or mounted. A fusible element or fuse link is disposed in and spaced from the inner bore of the bushing and is electrically connected between the terminal means adjacent to the opposite ends of the bushing. An insulating support may also be disposed in and spaced from the inner bore of the bushing and extend axially between the terminal means on the bushing with the fusible element mounted or supported on the insulating support where provided. The space between the fusible element (and its associated insulating support where provided) and the inner bore of the bushing which also functions as a housing or insulating tube for the fusible element is substantially filled with a pulverulent or granular arc quenching material, such as silica sand. The bushing also includes at its outer periphery axially intermediate its ends a portion which is adapted to receive a supporting means, such as the wall or cover of the associated electrical apparatus.

In an electrical apparatus embodying the teachings of the invention, the bushing described above would therefore pass through an opening in the wall or cover or other barrier of the enclosure of the apparatus and be supported intermediate the ends of the bushing with a first portion of the bushing having a terminal means mounted thereon and projecting away from the enclosure in one direction which would normally be outwardly from the enclosure and a second portion of the bushing having a terminal means mounted thereon and projecting away from the enclosure in the opposite direction which would normally be inwardly from the associated enclosure. Where the second portion of the bushing is disposed inside the enclosure of the electrical apparatus, electrical leads or conductors would normally be electrically connected between the terminal means on the second portion of the bushing and the portion of the electrical apparatus, such as an electrical winding, which is disposed inside the enclosure. The first or outer portion of the bushing just mentioned would provide sufficient electrical insulation between the terminal means of the electrical apparatus mounted thereon and the wall or cover of the enclosure which is usually metallic to withstand the potential applied between such terminal means and the wall or cover of the enclosure of the electrical apparatus. Where the enclosure of the electrical apparatus is at least partially filled with a dielectric fluid, such as an insulating oil, into which the second portion of the bushing projects or is immersed, it is preferable that a means for sealing the inner bore of the bushing adjacent the terminal means on the second portion of the bushing be provided to prevent the entrance of the dielectric fluid into the inner bore of the bushing.

It is therefore an object of this invention to provide an electrical apparatus including an improved protective device which is supported on a wall or enclosure of the electrical apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a view, partly in side elevation and partly in section, of a portion of a transformer including a protective device embodying the invention; and

FIG. 2 is a view in section, taken along the line Il-ll of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings and FIG. I, in particular, there is illustrated a portion of an electrical apparatus, more specifically, a transformer 10, including a protective device 20 embodying the principal features of the invention. As illustrated, the transformer 10 includes at least one electrical winding 18 which is disposed in inductive relation with an associated magneticcore 19. The winding 18 may be the only winding of the transformer 10 where the transformer 10 is an autotransformer or the winding 18 may be a high voltage winding and an additional low voltage winding may also be disposed in inductive relation with the magnetic core 19. The electrical winding 18 and the magnetic core 19 are disposed inside an associated enclosure or casing 14 which includes a cover 12. As shown in FIG. 1, the enclosure or housing 14 may be at least partially filled with a cooling dielectric fluid, such as an insulating oil, as indicated at 16 in FIG. I. The enclosure 14, as well as the associated cover 12, may be formed from an electrically conducting metallic material, such as steel and in certain applications may be maintained at or connected to ground potential.

In order to carry electrical current to the electrical winding 18 from a source of electrical power or alternating current voltage which is external to the enclosure 14 of the transformer 10, the protective device 20 is mounted on the cover 12 of the transformer 10 and is disposed to pass through the cover 12 as shown in FIG. 1. More specifically, the protective device 20 includes a hollow, generally cylindrical electrically insulating member or bushing 30 having an axially extending inner bore, as indicated at 34. The electrically insulating member 30 may be formed from a suitable insulating material of the ceramic type, such as porcelain, or may be cast or molded from a thermosetting resin, such as an epoxy or polyester resin, with a suitable filler, such as aluminum trihydrate, for antitracking characteristics. In order to mount the electrically insulating member 30 on the cover 12 at a location which is axially intermediate or spaced from the ends of the insulating member 30, the insulating member 30 includes at its outer periphery a portion which is axially intermediate the ends of said insulating member and which is adapted to receive an associated supporting means, such as the cover 12. More specifically, as illustrated the electrical insulating member 30 includes a groove or recess 30C adjacent to the cover 12 which is adapted to receive a ring shaped metallic member 33 which may be assembled into the groove or recess 30C. The ring shaped member 33 in turn may be welded or secured to the cover 12 by any suitable means around the opening 17 in the cover 12.

It is important to note that when the electrically insulating member 30 is mounted on the cover 12, as shown in FIG. 1, the insulating member 30 includes a first portion 30A which projects generally away from the cover 12 of the enclosure 14 and which may include a plurality of axially spaced petticoats or projecting portions 32 for outdoor applications to increase the electrical creepage distance along the upper portion 30A of the electrically insulating member 30. The lower or second portion of the insulating member 30, as indicated at 30B in FIG. 1, projects generally inwardly from the cover 12 and may project into or be immersed in a dielectric fluid, such as the oil indicated at 16.

In order to facilitate the assembly of an upper terminal member 42 as shown in the drawing, the upper end of the insulating member 30 includes a recess which extends around the inner bore 42 and which includes a radial slot 39 which is adapted to receive a cross pin 64 which is secured to or formed integrally with the terminal member 42. The cross pin 64 prevents rotation of the terminal member 42 and certain associated parts which will be described hereinafter after such parts are assembled with the insulating member 30. The upper terminal member 42 may be formed from any suitable electrically conducting material, such as copper, and, as illustrated, includes an externally threaded portion which projects or extends axially from the upper end of the insulating member 30. A solderless type connector 72 may be secured to the upper terminal member 42 by screwing an internally threaded portion of the connector 72 on the axially extending, threaded portion of the upper terminal member 42, as shown in the drawing. The connector 72 may include an overhanging portion which seats against the upper end of the insulating member 30 and closes off the inner bore of the insulating member at the upper end of said insulating member. As shown in the drawing, the connector 72 includes an adjustable threaded member to permit electrical connections to the protective device from an external source of electrical power or alternating current voltage. In order to seal off the upper end of insulating member 30, a gasket member 62 may be disposed between an overhanging portion of the connector 72 and the upper end of insulating member 30 and compressed when the connector 72 is tightened down on the upper terminal member 42.

In order to permit the making of electrical connections to the lower end of the protective device from the electrical winding 18 as indicated in FIG. 1, the lower terminal member 44 is mounted or disposed adjacent to the lower end of the electrically insulating member 30. As illustrated, the terminal member 44 includes a threaded stud portion which projects axially downward and on which a centering or spacer member 46 may be assembled and retained on the threaded terminal member between a shoulder portion of the terminal member 44 and a nut which is assembled on the other side of the centering member 46. The shape of the centering member 46 may be generally circular or triangular in a particular application. In order to hermetically seal off the lower end of the inner bore 34 of the insulating member and to assist in retaining the terminal member 44 in its proper assembled position, a suitable means, such as a thermosetting resin of the epoxy type as indicated at 73 in FIG. 1, may be disposed in the inner bore 34 of the insulating member 30 to surround-or embed the lower terminal member 44 as indicated in FIG. 1. The electrical conductors or leads 13 may be provided between the electrical winding 18 and the lower end of the terminal member 44 to electrically connect the electrical winding 18 to the terminal member 44.

In order to provide an electrically conducting path between the upper and lower terminal members 42 and 44, respectively, and to protect the electrical winding 18 or other portions of an electrical apparatus which may be disposed inside the enclosure 14, a fusible element or fuse link 52 is electrically connected between the upper and lower terminal members 42 and 44 respectively. The fusible element 52 may be of the flat strip or ribbon type and is preferably composed of silver. The fusible element 52 includes a plurality of axially spaced notches or restricted cross-sectional areas as indicated at 54 in FIG. 1 which may be V-shaped in configuration to provide a series of arcs during the operation of the protective device with the sum of the arc voltages providing a current limiting effect which is known in the fuse art. As illustrated, the fusible element 52 is helically wound on an associated electrically insulating support member 82 which extends axially between the upper and lower terminal members 42 and 44, respectively. The opposite ends of the fusible member or element 52 may be electrically connected and secured to the associated terminal members 42 and 44 .by suitable means, such as brazing. In order to aid in arc extinction during the operation of the protective device 20, the insulating support member 82 may be molded or cast from a suitable electrically insulating material which also evolves gasses during a circuit interruption which aid in arc extinction, such as a glass-polyester material, with a suitable filler, such as aluminum trihydrate, for antitracking characteristics. As shown in FIG. 2 the insulating support member 82 may be substantially rectangular in cross section.

In order to additionally aid in arc extinction during the operation of the protective device 20 and to provide the current limiting action which is particularly desirable in protecting certain types of electrical apparatus, the space between the inner bore 34 of the insulating member 30 and the fusible member 52 or between the inner bore 34 and the combination which includes the fusible element 52 and the associated insulating support 82, is substantially filled with a pulverulent or granular arc quenching material, such as silica sand. It is important to note that the arc quenching material 92 is disposed in direct heat transferring relationship between the fusible element 52 and the inner bore or surface 34 of the insulating member 30 and provides a continuous heat flow path from said fusible element to said insulating member. The presence of the arc quenching material or filler 92 is also important in preventing the occurrence of corona due to the presence of voids or air pockets which may result when a conventional fuse having its own insulating housing assembled inside a bushing as in the structures of the patents previously mentioned. It is also important to note that the electrically insulating member 30 in the construction of the protective device 20 as just described also performs the function of an electrically insulating housing for the fusible element 52, the insulating support 82 where provided, and the arc quenching material 92, as well as providing the necessary electrical insulation required between the upper terminal member 42 and the cover 12 of enclosure 14 considering the bushing function of the protective device 20. As illustrated, the electrically insulating member 30 also provides electrical insulation which cooperates with the insulating oil 16, as illustrated, to provide the necessary electrical insulation between the lower terminal 44 and the enclosure 14 which includes the cover 12. In other words, the electrical insulation provided by the portion of the insulating member 30 between the upper terminal 42 and the cover 12 is sufficient to withstand the potential applied between the upper terminal 42 and the cover 12 of the enclosure 14 as previously described in connection with the upper portion 30A'of the insulating member 30.

In the operation of the protective device 20, when the current flowing through the protective device 20 to the portions of the electrical'apparatus inside the associated enclosure 14 increases to an abnormal value, the fusible element 52 will fuse or melt at one or more of the plurality of constricted portions 54 to result in a plurality of arc voltages which will effectively limit the current through the protective device to a value, which is referred to as the let-through current" and which is normally much less than the available fault current which might otherwise result. The pulverulent material in the inner bore 34 of the insulating member 30 cooperates with the fusible element 52 during an interrupting operation to form a fulgurite as described in greater detail in U.S. Pat. Nos. 2,496,704 and 2,502,992.

The thermal mass of the insulating member 30 which is much greater than that of a conventional fuse structure provides desirable short-time overload capability in the protective device 20 which more closely matches and coordinates with the thermal capability and characteristics of the electrical apparatus it protects. The insulating member 30 is also much more effective in continuous heat dissipation than that which is possible in known fuse structures because of (1) its increased heat radiating surface due to the presence of the axially spaced petticoats 32, (2) the high temperature characteristics of the electrically insulating material from which the insulating member 30 is formed, (3) the utilization of the metallic enclosure or tank of the associated electrical apparatus as a heat sink, and (4) the lowerthermal impedance from the fusible elements where heat is generated to the insulating member compared with that in known fuse struc- 1 tures. The lower thermal impedance in the applicants invention results from a continuous heat flow path of substantially solid material from each fusible element to the insulating member 30 whereas known devices of the same general type include an air gap in the corresponding heat flow path.

In the assembly of the protective device 20, the upper and lower terminal members 42 and 44, respectively, which are formed from any electrically conducting material, such as copper, may be preferably molded to the opposite ends of the insulating support member 82 where such a support member is provided. The fusible element 52 may then be assembled on the support member 82 by helically winding the fusible element 52 on the support member 82 and then securing the upper and lower ends of the fusible element 52 to the associated upper and lower terminal members 42 and 44, respectively, by suitable means, such as brazing. The pin 64 may then be inserted into the upper threaded terminal member 42 and the centering member 46, as well as the associated nut, may be assembled on the lower threaded terminal member 44 as shown in FIG. 1. The sub-assembly which includes the fusible element 52 and the support member 82 may then be assembled into the lower end of the inner bore 34 of the insulating member 30 as viewed in FIG. 1 with the pin 64 being positioned within the slot 39 provided in the insulating member 30 to prevent rotation of the sub-assembly in place after the protective device 20 is completely assembled. After the fusible element 52 and the associated support member 82 are assembled in the inner bore .34 of the insulating member 30 as shown in FIG. 1, the insulating member 30 may be sealed at the upper end by the assembly of the gasket member 62 as previously described. It is to be noted that the sealing of the upper end of the insulating member 30 prevents the entrance of moisture or other contaminating materials into the upper end of the inner bore 34 of the insulating member 30 of the protective device 20 during long periods of service in outdoor applications. After the upper end of the insulating member 30 is sealed, the insulating member 30 may be positioned with the opening of the inner bore adjacent to the terminal member 44 substantially reversed from the position shown in FIG. 1 with the opening around the terminal member 44 being at the upper end of the insulating member to permit the filling of the inner bore 34 of the insulating member 30 with the arc quenching material 92 which may then be compacted by any suitable means, such as vibration or other methods. After the arc quenching material 92 is in position inside the inner bore of the insulating member 30, the thermosetting resin which forms the sealing member 73 and which includes any desired or suitable filler may be poured in the opening around the terminal member 44 to cure or polymerize in place to complete the assembly of the protective device 20. As previously mentioned, the sealing means indicated at 73 substantially prevents the entrance of any fluid dielectric or insulating liquid, such as oil, which might adversely effect the operating characteristics of the protective device quantity.

It is to be understood that the teachings of the applicants invention may be applied to a protective device which performs the combined function of an electrically insulating bushing and a protective fuse as disclosed and which may be disposed or mounted on the enclosure of other types of electrical apparatus, such as a circuit breaker or enclosed electric switch.

The apparatus and the protective device embodying the teachings of this invention have several advantages. For example, an electrical apparatus including a protective device as disclosed is relatively more compact since the electrical bushing or insulating member as disclosed eliminates the need for a separate insulating bushing'and an insulating housing which forms part of a separate fuse device which may be assembled inside the bushing. In addition, utilizing the inner bore of an electrically insulating member as disclosed results in a relatively stronger housing for the fuse parts which are disposed inside the bushing and increases the effective thermal mass which is available to determine the thermal operating characteristics of the protective device as disclosed. Another important advantage of the disclosed construction is that the presence of voids or air pockets is substantially eliminated between the assembled parts to substantially eliminate the possibility of corona, particularly in relatively higher voltage applications, which might otherwise result in conventional structures of the same general type. Finally, in the disclosed construction of a protective device only a portion of the electrically insulating member 30 is sufficient to support the potential applied between one of the terminal means and the associated metallic cover or housing to substantially avoid the possibility of a flashover along the electrically insulating member considered as a housing for the associated fuse parts. A further advantage of the disclosed construction is that it is uniquely adapted to a protective device which combines the functions of an electrical bushing and a protective device having current limiting characteristics and more nearly matches or coordinates with the thermal characteristics of the electrical apparatus being protected.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit and the scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim:

1. A high voltage current limiting electrical bushing comprising a generally hollow cylindrical, electrically insulating member having an axially extending inner bore, first and second terminal means mounted on said insulating member adjacent to the opposite ends of said electrically insulating member, a fuse link disposed in said inner bore and laterally spaced from said inner bore, said fuse link being electrically connected between said first and second terminal means and including a plurality of axially spaced restricted cross-sectional areas, a pulverulent, arc quenching material substantially filling the space between said fuse link and the inner bore of said electrically insulating member and being disposed in heat transferring relation with both said fuse link and the inner bore of said insulating member to provide a heat flow path from said fuse link to said insulating member through said pulverulent arc quenching material, said electrically insulating member including a predetermined portion along its outer periphery axially spaced from both ends of said electrically insulating member and adapted to receive a metallic supporting means for said bushing, the portion of said electrically insulating member between said first terminal member and said predetermined portion of said electrically insulating member including a plurality of axially spaced, outwardly projecting portions which increase the heat radiating surface of said insulating member which is effective when heat flows from said fuse link to said insulating member through said pulverulent arc quenching material.

2. The combination as claimed in claim 1, wherein the portion of said electrically insulating member between said first terminal means and said predetermined portion of said electrically insulating member is sufficient to withstand a predetermined potential applied between said first terminal means and the supporting means associated with said bushing.

3. The combination as claimed in claim 1, wherein an insulating support is disposed in and spaced fromsaid inner bore to extend axially between said terminal means, said fuse link being mounted on said insulating support, and said pulverulent material substantially filling the space between said inner bore and the combination which includes said fuse link and said insulating support.

4. The combination as claimed in claim 1, wherein a means is disposed adjacent to the terminal means at one end of said bushing for sealing off at least one end of said inner bore and said pulverulent, arc quenching material comprises silica sand.

5. The combination as claimed in claim 1, wherein said fuse link is composed of silver, and said pulverulent arc quenching material comprises silica sand which forms a fulgurite during an interrupting operation.

6. A current limiting protective device for an electrical apparatus comprising a hollow, generally cylindrical, electrically insulating bushing having an axially extending inner bore, terminal means disposed adjacent to each end of said bushing, a fuse element disposed in and spaced from said inner bore, said fuse element being electrically connected between said terminal means and including a plurality of axially spaced restricted cross-sectional area, a pulverulent, arc quenching material substantially filling the space between said fuse element and said inner bore and being disposed in heat transferring relation with both said fuse element and the inner bore of said bushing to provide a heat flow path from said fuse element to said bushing through said pulverulent arc quenching material, and means on the outer periphery and axially intermediate the ends of said bushing for mounting said bushing on a supporting barrier at least the portion of said insulating bushing between the last-mentioned mounting means and said terminal means at one end of said bushing including a plurality of axially spaced, outwardly projecting portions which increase the heat radiating surface of said insulating bushing which is effective when heat flows from said fuse element to said bushing through said pulverulent arc quenching material.

7. The combination as claimed in claim 6, wherein an insulating support is disposed in and spaced from said inner bore to extend axially between said terminal means, said fuse element being mounted on said insulating support.

8. The combination as claimed in claim 6 wherein a means is disposed adjacent to the terminal means at one end of said bushing for sealing off one end of said inner bore.

9. The combination as claimed in claim 6, wherein the portion of said electrically insulating member between said first terminal means and said predetermined portion of said electrically insulating member is sufficient to withstand a predetermined potential applied between said first terminal means and the supporting means associated with said bushing.

10. The combination as claimed in claim 6 wherein said fuse element is composed of silver, and said pulverulent arc quenching comprises silica sand.

11. An electrical apparatus comprising a metallic housing, means disposed in said housing for normally carrying electric current, a generally hollow cylindrical, electrically insulating bushing having an axially extending inner bore, said bushing being disposed to pass through and to be supported by said housing at a location axially intermediate the ends of said bushing, saidbushing including a first portion which projects outside of said housing and a second portion WhlCl'l pro ects inside said housing, said first portion of said bushing including a plurality of axially spaced, outwardly projecting portions which increase the heat radiating surface of said bushing which is effective when heat flows to said bushing, first and second terminals mounted on said bushing adjacent to the opposite ends of said bushing outside and inside, respectively, of said housing, a fuse element disposed in the inner bore of said bushing and laterally spaced from said inner bore, said fuse element being electrically connected between said first and second terminal means and including a plurality of axially spaced, restricted cross-sectional areas, a pulverulent, arc quenching material substantially filling the space between said fuse element and the inner bore of said bushing and being disposed in heat transferring relation with both said fuse element and the inner bore of said bushing to provide a heat flow path from said fuse element to said bushing through said pulverulent arc quenching material, and electrical leads connected between said second terminal means and said firstmentioned means for carrying electric current to and from said first-mentioned means.

12. The combination as claimed in claim 11, wherein said fuse element is composed of silver and said pulverulent arc quenching material comprises silica sand.

13. The combination as claimed in claim 11, wherein the first-mentioned means comprises an electrical winding.

14. The combination as claimed in claim 11, wherein said housing is at least partially filled with a dielectric fluid, said bushing including a portion which is immersed in said dielectric fluid adjacent to said second terminal means, and means is disposed adjacent to said second terminal means to substantially seal the inner bore of said bushing to thereby substantially prevent the entrance of said dielectric fluid into said inner bore. 

1. A high voltage current limiting electrical bushing comprising a generally hollow cylindrical, electrically insulating member having an axially extending inner bore, first and second terminal means mounted on said insulating member adjacent to the opposite ends of said electrically insulating member, a fuse link disposed in said inner bore and laterally spaced from said inner bore, said fuse link being electrically connected between said first and second terminal means and including a plurality of axially spaced restricted cross-sectional areas, a pulverulent, arc quenching material substantially filling the space between said fuse link and the inner bore of said electrically insulating member and being disposed in heat transferring relation with both said fuse link and the inner bore of said insulating member to provide a heat flow path from said fuse link to said insulating member through said pulverulent arc quenching material, said electrically insulating member including a predetermined portion along its outer periphery axially spaced from both ends of said electrically insulating member and adapted to receive a metallic supporting means for said bushing, the portion of said electrically insulating member between said first terminal member and said predetermined portion of said electrically insulating member including a plurality of axially spaced, outwardly projecting portions which increase the heat radiating surface of said insulating member which is effective when heat flows from said fuse link to said insulating member through said pulverulent arc quenching material.
 2. The combination as claimed in claim 1, wherein the portion of said electrically insulating member between said first terminal means and said predetermined portion of said electrically insulating member is sufficient to withstand a predetermined potential applied between said first terminal means and the supporting means associated with said bushing.
 3. The combination as claimed in claim 1, wherein an insulating support is disposed in and spaced from said inner bore to extend axially between said terminal means, said fuse link being mounted on said insulating support, and said pulverulent material substantially filling the space between said inner bore and the combination which includes said fuse link and said insulating support.
 4. The combination as claimed in claim 1, wherein a means is disposed adjacent to the terminal means at one end of said bushing for sealing off at least one end of said inner bore and said pulverulent, arc quenching material comprises silica sand.
 5. The combination as claimed in claim 1, wherein said fuse link is composed of silver, and said pulverulent arc quenching material comprises silica sand which forms a fulgurite during an interrupting operation.
 6. A current limiting protective device for an electrical apparatus comprising a hollow, generally cylindrical, electrically insulating bushing having an axially extending inner bore, terminal means disposed adjacent to each end of said bushing, a fuse element disposed in and spaced from said inner bore, said fuse element being electrically connected between said terminal means and including a plurality of axially spaced restricted cross-sectional area, a pulverulent, arc quenching material substantially filling the space between said fuse element and said inner bore and being disposed in heat transferring relation with both said fuse element and the inner bore of said bushing to provide a heat flow path from said fuse element to said bushing through said pulverulent arc quenching material, and means on the outer periphery and axially intermediate the ends of said bushing for mounting said bushing on a supporting barrier at least the portion of said insulating bushing between the last-mentioned mounting means and said terminal means at one end of said bushing including a plurality of axially spaced, outwardly projecting portions which increase the heat radiating surface of said insulating bushing which is effective when heat flows from said fuse element to said bushing through said pulverulent arc quenching material.
 7. The combination as claimed in claim 6, wherein an insulating support is disposed in and spaced from said inner bore to extend axially between said terminal means, said fuse element being mounted on said insulating support.
 8. The combination as claimed in claim 6 wherein a means is disposed adjacent to the terminal means at one end of said bushing for sealing off one end of said inner bore.
 9. The combination as claimed in claim 6, wherein the portion of said electrically insulating member between said first terminal means and said predetermined portion of said electrically insulating member is sufficient to withstand a predetermined potential applied between said first terminal means and the supporting means associated with said bushing.
 10. The combination as claimed in claim 6 wherein said fuse element is composed of silver, and said pulverulent arc quenching comprises silica sand.
 11. An electrical apparatus comprising a metallic housing, means disposed in said housing for normally carrying electric current, a generally hollow cylindrical, electrically insulating bushing having an axially extending inner bore, said bushing being disposed to pass through and to be supported by said housing at a location axially intermediate the ends of said bushing, said bushing including a first portion which projects outside of said housing and a second portion which projects inside said housing, said first portion of said bushing including a plurality of axially spaced, outwardly projecting portions which increase the heat radiating surface of said bushing which is effective when heat flows to said bushing, first and second terminals mounted on said bushing adjacent to the opposite ends of said bushing outside and inside, respectively, of said housing, a fuse element disposed in the inner bore of said bushing and laterally spaced from said inner bore, said fuse element being electrically connected between said first and second terminal means and including a plurality of axially spaced, restricted cross-sectional areas, a pulverulent, arc quenching material substantially filling the space between said fuse element and the inner bore of said bushing and being disposed in heat transferring relation with both said fuse element and the inner bore of said bushing to provide a heat floW path from said fuse element to said bushing through said pulverulent arc quenching material, and electrical leads connected between said second terminal means and said first-mentioned means for carrying electric current to and from said first-mentioned means.
 12. The combination as claimed in claim 11, wherein said fuse element is composed of silver and said pulverulent arc quenching material comprises silica sand.
 13. The combination as claimed in claim 11, wherein the first-mentioned means comprises an electrical winding.
 14. The combination as claimed in claim 11, wherein said housing is at least partially filled with a dielectric fluid, said bushing including a portion which is immersed in said dielectric fluid adjacent to said second terminal means, and means is disposed adjacent to said second terminal means to substantially seal the inner bore of said bushing to thereby substantially prevent the entrance of said dielectric fluid into said inner bore. 